The invention relates to an optical waveguide fiber. In particular, the core to clad refractive index contrast in the waveguide fiber is achieved by incorporating a photonic-crystal-like structure into the fiber clad layer.
Waveguide fibers having a photonic crystal clad layer have been described in the literature. At present the photonic crystal fiber (PCF) includes a porous clad layer, i.e., a clad layer containing an array of voids that serves to change the effective refractive index of the clad layer, thereby changing the properties of the waveguide fiber such as mode field diameter or total dispersion. The distribution of light power across the waveguide (mode power distribution) effectively determines the properties of an optical waveguide. Changing the effective index of the clad layer changes the mode power distribution and thus the waveguide fiber properties.
In addition to the properties set forth above, the cut off wavelength is also affected by the clad layer structure is cut-off wavelength. An advantageous feature of a porous clad PCF is that a particular choice of pore size and pore distribution in the clad layer results in the fiber transmitting a single mode for signals having essentially any wavelength. That is the wavelength span of the cut off wavelength is large without bound. Such a PCF has been denoted "endlessly single mode". An additional benefit afforded by the PCF is the availability of high contrast in refractive index between core and clad at dopant levels near to or lower than the levels in non-PCF waveguide fiber.
The manufacture of a porous clad PCF is difficult because the porosity volume and distribution must be controlled in the preform. Further, the control of the PCF clad porosity must be maintained during drawing of the preform down to the dimensions of a waveguide fiber. Higher speed drawing does reduce manufacturing cost, which means that present PCF drawing processes increases factory cost. The drawing step occurs at very high temperatures and the final fiber diameter is small, about 125 .mu.m. The drawing step must therefore include the maintaining a precise balance of pressure within the pore against viscous forces of the material surrounding the pore under relatively extreme conditions.
It is expected that the porous clad PCF will be susceptible to OH.sup.- contamination because at least a portion of the light carrying area of the fiber has a relatively large surface area open to atmosphere after the OH.sup.- removal step. The OH.sup.- removal step, known in the art, usually includes treating the heated preform with a reactive gas such as chlorine. An example of OH.sup.- contamination is shown in curve 2 of FIG. 1. The overall attenuation is high, being above about 20 dB/km over the wavelength range 800 nm to 1600 nm. In addition the OH.sup.- absorption peak 4 at 1250 nm, and, the local maxima 6 and 8, which characterize the broad OH.sup.-- maximum from about 1390 nm to 1450 nm, are unacceptably high and essentially render the waveguide useless except perhaps in very short length applications.
The endlessly single mode property is however of sufficient value to attract workers to address the problem of PCF manufacture. Another incentive to develop a reliable and reproducible process for the PCF is the possibility of achieving unusual dispersion properties which can be used for example in dispersion compensating fiber. The dispersion compensating fiber compensates the dispersion in an existing communication link, thereby allowing operation of the link at a different wavelength. Another PCF advantage is that the large contrast available between core and clad effective index can be used to provide large effective area, thereby mitigating non-linear effects on transmitted signal integrity.
The present waveguide fiber and waveguide fiber preform disclosed and described herein reduces the unsatisfactory OH.sup.- contamination and effectively overcomes the problems in the prior art.